1. Field of the Invention
The present invention relates to a printing system, which has a head for recording three or more different inks that are mixable to express hues in a predetermined range and records multi-tone images with the inks discharged from the head onto a printing object. The present invention also pertains to a method of recording images and an ink cartridge attachable to such a printing system.
2. Description of the related Art
Color printers with a head for recording a plurality of color inks on a printing object are widely used as an output device of a computer, which prints multi-color, multi-tone images processed by the computer. The following techniques are known as the method of recording inks on a printing object: a thermal transfer system that melts inks on ink ribbons and transfers the molten inks to a sheet of paper; an ink jet system that sprays colored ink solutions onto a sheet of paper; and an electrophotographic system that uses laser or another light to create a latent image on a photoconductive intermediary and transfers color toners to render the latent image visible. All such available techniques reproduce colors in a predetermined hue range by mixing a plurality of color inks. Three color inks, cyan, magenta, and yellow (CMY) are generally used for full-color printing.
There are several techniques available for printing multi-tone, multi-color images with a plurality of color inks. One available technique, which is adopted in the conventional printers, expresses the tone of a printed image by the density of dots (the frequency of appearance of dots per unit area) while fixing the size of dots formed on a sheet of paper by a stream of ink droplets. Another available technique adjusts the diameter of dots formed on a sheet of paper, in order to vary the density of ink per unit area. The advanced fine working of the head for creating ink particles has improved the density of dots formable per predetermined length or the variable range of dot diameters.
The improvement in printers has, however, so far been limited to 300 dpi through 720 dpi in printing density or resolution and several tens microns in particle diameter. This is significantly lower than the level of expression or resolution of silver photography, which has reached several thousands dpi on the film. Dots are sparsely formed in an area of low image density, that is, in an area of low density of dots to be printed. This increases the degree of granularity and makes the dots undesirably conspicuous. In printers for spraying liquid ink onto a paper, the total amount of ink sprayed per unit area is restricted by the absorbable volume of ink by the paper (generally referred to as ink duty). The restriction of ink duty is an issue to be cleared in printers using a plurality of color inks for color printing. The issue of ink duty is actualized especially in case that higher-density ink and lower-density ink are provided for the respective colors and the lower-density ink is used for printing low-tone areas in order to decrease the degree of granularity. Expression of a specific tone with the lower-density ink results in increasing the total amount of ink sprayed per unit area.
The object of the present invention is thus to regulate the density of a specific color ink and relieve the restriction of ink duty while keeping the quality of a printed image in a printing system with a head for spraying three or more different inks that are mixable to express hues in a predetermined range.
At least part of the above and the other related objects are realized by a first printing system having a head for recording at least three different color inks, which are mixable to express hues in a predetermined range, on a printing object, wherein a density ratio of a specific color ink, which has highest lightness against a fixed recording ratio among the at least three different color inks, to the other color inks is determined, in order to enable a color balance to be biased to the specific color ink having the highest lightness in case that the specific color ink and the other color inks have an identical recording ratio per unit area. The first printing system includes correction means for correcting a recording amount of the specific color ink having the highest lightness to a level that cancels the bias.
The present invention is also directed to a first image recording method corresponding to the first printing system. The first method records a multi-tone image by controlling a head that is able to record at least three different color inks, which are mixable to express hues in a predetermined range, and regulating a distribution of dots formed by the at least three different color inks based on tone signals regarding an original image to be printed. The first method includes the steps of:
determining a density ratio of a specific color ink, which has highest lightness against a fixed recording ratio among the at least three different color inks, to the other color inks, in order to enable a color balance to be biased to the specific color ink having the highest lightness in case that the specific color ink and the other color inks have an identical recording ratio per unit area; and
correcting a recording amount of the specific color ink having the highest lightness to a level that cancels the bias.
The first printing system (or the corresponding first image recording method) of the present invention has a head for recording at least three different color inks that are mixable to express hues in a predetermined range. The printing system forms dots of these color inks at a predetermined ratio, so as to record images of various hues and lightness (densities). In the first printing system, the color balance is biased to the specific color ink having highest lightness against a fixed recording ratio, in case that all the color inks have an identical recording ratio per unit area. The correction means then corrects the recording amount of the specific color ink having the highest lightness to a level that cancels the bias. Such correction ensures a normal color balance and reduces the recording amount of the specific color ink having the highest lightness and less effect of granularity in a low density area. This structure accordingly reduces the total amount of inks to be recorded while keeping the quality of a resulting image.
At least part of the above and the other related objects are also realized by a second printing system having a head for recording at least three different color inks, which are mixable to express hues in a predetermined range, on a printing object, wherein a density ratio of a specific color ink, which has lowest conspicuousness of granularity against a fixed recording ratio among the at least three different color inks, to the other color inks is determined, in order to enable a color balance to be biased to the specific color ink having the lowest conspicuousness of granularity in case that the specific color ink and the other color inks have an identical recording ratio per unit area. The second printing system includes correction means for correcting a recording amount of the specific color ink having the lowest conspicuousness of granularity to a level that cancels the bias.
The present invention is further directed to a second image recording method corresponding to the second printing system. The second method records a multi-tone image by controlling a head that is able to record at least three different color inks, which are mixable to express hues in a predetermined range, and regulating a distribution of dots formed by the at least three different color inks based on tone signals regarding an original image to be printed. The second method includes the steps of:
determining a density ratio of a specific color ink, which has lowest conspicuousness of granularity against a fixed recording ratio among the at least three different color inks, to the other color inks, in order to enable a color balance to be biased to the specific color ink having the lowest conspicuousness of granularity in case that the specific color ink and the other color inks have an identical recording ratio per unit area; and
correcting a recording amount of the specific color ink having the lowest conspicuousness of granularity to a level that cancels the bias.
The second printing system (or the corresponding first image recording method) of the present invention has a head for recording at least three different color inks that are mixable to express hues in a predetermined range. The printing system forms dots of these color inks at a predetermined ratio, so as to record images of various hues and lightness (densities). In the second printing system, the color balance is biased to the specific color ink having lowest conspicuousness of granularity against a fixed recording ratio, in case that all the color inks have an identical recording ratio per unit area. The correction means then corrects the recording amount of the specific color ink having the lowest conspicuousness of granularity to a level that cancels the bias. Such correction ensures a normal color balance and reduces the recording amount of the specific color ink having the lowest conspicuousness of granularity. This structure accordingly reduces the total amount of inks to be recorded while keeping the quality of a resulting image.
Three primary color inks, that is, yellow, magenta, and cyan inks, are practically adopted as the at least three different color inks in the first and the second printing systems. It is also practical to select yellow as the specific color ink having the highest lightness or the lowest conspicuousness of granularity. In another combination of color inks, the specific color ink having highest lightness or lowest conspicuousness of granularity should be selected among all the color inks.
There are a variety of approaches to bias the color balance in the specified combination of color inks. One possible approach increases the dye density of the specific color ink having the highest lightness or the lowest conspicuousness of granularity in a range of 1.1 to 4 times a balancing density that ensures a color balance in case that the at least three different color inks have an identical recording ratio per unit area. The increased density of less than 1.1 times can not exert the expected effects, whereas the increased density of greater than 4 times worsens the degree of granularity. Simple adjustment of the dye density can readily bias the color balance to a desired extent.
Correction of the recording amount of the specific color ink having the highest lightness or the lowest conspicuousness of granularity may be accomplished by decreasing the proportion of dots formed by the specific color ink or by decreasing the diameter of dots formed by the specific color ink.
Another possible approach to bias the color balance in the specified combination of color inks sets the diameter of dots formed by the specific color ink having the highest lightness or the lowest conspicuousness of granularity to be greater than the diameter of dots formed by the other color inks. In this case, the correction means carries out the required correction by decreasing the proportion of dots formed by the specific color ink.
A variety of known methods are applicable to record at least three different color inks on a printing object. By way of example, each of these color inks is provided as a solution prepared by dissolving or dispersing a dye or a pigment in a solvent, and each solution containing the dye or the pigment is sprayed from the head onto the printing object.
In this structure, the correction means corrects the discharge amount of the specific color ink. The method of spraying the ink solution advantageously forms fine dots at a relatively high speed.
In the printing system using such ink solutions, it is referable that at least two types of inks having different densities are provided for the color inks other than the specific color ink among the at least three different color inks. In this structure, the specific color ink having the highest lightness or the lowest conspicuousness of granularity as well as the at least two types of inks having different densities with respect to the other color inks are sprayed from the head. This structure provides the low-density, light ink for the colors having relatively high conspicuousness of granularity, thereby preventing an increase in degree of granularity in a low density area.
In the color combination of yellow, magenta, and cyan, the at least two types of inks having different densities are provided for magenta and cyan. The dye density of the lower-density ink with respect to each color is approximately one quarter the dye density of the higher-density ink. This ensures natural effects on a density variation in the place where dots of the deep ink are mixed with dots of the light ink.
A variety of techniques are applicable to form dots of the respective color inks. For example, the dither method may be applied to determine formation or non-formation of dots by each color ink. In the dither method, a threshold matrix of discrete dither can be used to determine the on/off state of dots. The threshold matrix of discrete dither well disperses the dots and thus advantageously improves the degree of granularity.
A variety of mechanisms are applicable for dot formation in such printing systems. By way of example, the head may have a mechanism for discharging ink particles under a pressure applied to each ink running through an ink conduit by application of a voltage to a piezoelectric element arranged in the ink conduit, or a mechanism for discharging ink particles under a pressure applied to each ink running through an ink conduit by air bubbles that are produced by a supply of electricity to a heating body arranged in the ink conduit.
The present invention is further directed to an ink cartridge attachable to either one of the above printing systems or used in either one of the above methods, which has a head for recording at least three different color inks, which are mixable to express hues in a predetermined range, on a printing object. The ink cartridge includes the at least three different color inks, which are mixable to express hues in the predetermined range and stored therein. At least either one of a specific color ink having highest lightness against a fixed recording ratio and a specific color ink having lowest conspicuousness of granularity against the fixed recording ratio among the at least three different color inks has a dye density higher than a balancing density that ensures a color balance in case that the at least three different color inks have an identical recording ratio per unit area. A volume of the specific color ink stored in the ink cartridge is not greater than a volume of each of the other color inks.
In the printing systems and the corresponding image recording methods discussed above, the specific color ink having highest lightness or lowest conspicuousness of granularity against a fixed recording ratio has the increased dye density and thereby the decreased volume of consumption. The volume of the specific color ink stored in the ink cartridge is accordingly equal to or less than the volumes of the other color inks. This enables the respective color inks to be used up at substantially the same timings.
In case that at least two types of inks having different densities are provided for the color inks other than the specific color ink having the highest lightness or the lowest conspicuousness of granularity, each of the at least two types of inks having different densities has a less volume of consumption. In this case, the volume of the specific color ink stored in the ink cartridge may be set greater than the volume of each of the at least two types of inks having different densities.
These and other objects, features, aspects, and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiment with the accompanying drawings.